A synchronous machine is categorized as a permanent magnet motor and a reluctance motor, according to a fact whether a permanent magnet is included or not. Furthermore, a permanent magnet motor including a permanent magnet is categorized into an encased permanent magnet motor and a surface-mounted permanent magnet motor, depending on a method how to put the permanent magnet in a rotor.
Here, a permanent magnet motor is superior to an induction motor in its efficiency and output per volume. However, the permanent magnet motor is generally more expensive than the induction motor. Meanwhile, a reluctance motor without a permanent magnet is similar to an inductance motor in its price. However, the reluctance motor has no copper loss, because the current does not flow in a rotor of the reluctance motor. Therefore, efficiency of the reluctance motor is superior to that of the induction motor.
Meanwhile, an MTPA (Maximum Torque Per Ampere) operation may minimize the copper loss, because the machine is driven by minimum current at the same output, when considering only the copper loss excluding the iron loss, provided that a loss of a synchronous machine is categorized into a copper loss and an iron loss.
However, the conventional technology suffers from disadvantages in that an MTPV (Maximum Torque Per Voltage) curve and an initial value cannot be accurately measured and set.